The present invention relates to the color image processing arts. It finds particular application in conjunction with color digital half-toning, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other analogous applications.
Digital half-toning techniques are employed in image processing to simulate a continuous tone image with bi-level or otherwise quantized level devices. Examples of these techniques include fixed pattern, ordered dither, and error diffusion (ED). As well, for color applications such as ink jet printers and other digital color devices, color digital half-toning has been developed. Two such color digital half-toning techniques are scalar ED and vector ED.
In scalar ED, the different color components of the color image are separated out and processed monochromatically. That is to say, each color channel is processed independently as a one-dimensional ED problem. The quantized output for an object pixel is determined by comparing a modified input to a preset threshold. For a bi-level device this means that if the modified input was higher than the preset threshold the output would be high/on, if not the output would be low/off. The modified input represents an input value for the object pixel corresponding to the density of the color component being processed plus the sum of weighted quantization error from other quantized pixels in the neighborhood of the object pixel. The quantization error represents the difference between the quantized output and the modified input. In this manner, the error from the quantization of any one pixel is diffused to nearby surrounding pixels. Examples of such ED methods, are found in R. Floyd et al., “An Adaptive Algorithm for Spatial Greyscale,” Proceedings of the SID, (2nd Quarter 1976), Vol. 17/2, pg. 75, U.S. Pat. No. 5,353,127 to Shiau et al. and U.S. Pat. No. 5,521,989 to Fan, both incorporated herein by reference. One drawback of scalar ED for color digital half-toning is that the technique is performed independently for each color component and the colorimetric color reproduction is not guaranteed.
Vector ED is similar to scalar ED. The difference is that in vector ED the pixels are represented by vectors in a chosen color space and an error is formulated as a multi-dimensional function. The different color components are not separately processed in a one-dimensional scalar manner. Moreover, the chosen color space is typically related to certain perceptual measures such that an output is generated which produces the least perception error. Generally, vector ED is superior to scalar ED with regard to visual noise level and achieves better reproduction quality. However, vector ED suffers from its own inherent drawbacks as pointed out by H. Haneishi et al. in “Color Digital Halftoning Taking Colorimetric Color Reproduction into Account,” Journal of Electronic Imaging, (January 1996), Vol. 5(1), pg. 97. Vector ED tends to introduce artifacts at the leading and trailing edges of certain areas of color transitions. These artifacts have been termed slow response and smear respectively. Both the slow response and smear artifacts originate from the difference between the perceptual error and output error. In many instances, a minimum error in a perceptual space may imply a large error in an output device color space. Slow response results from a nonuniform distribution of primary colors. That is, contrary to ideal circumstances, not all the desired color components may initially participate in the reproduction resulting in slow response or a band of color without an ideal distribution. Smear results from the build up or accumulation of error being dumped into a second area upon a transition from a first area of a different color. In some cases, it can take many pixels to cancel the residual error that has built up. While the H. Haneishi et al. reference describes a technique for reduction of the smear artifact, it does not address or correct the slow response artifact.
The present invention contemplates a new and improved vector error diffusion technique which overcomes both the above-referenced slow response and smear problems and others.